10. Fair Value of Financial Instruments (Details 6) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Embedded Derivatives [Member]
Level 3 Assets and Liabilities by Pricing Source:
Total liabilities Internal	$ 79.5	$ 82.3
Total liabilities External
Total liabilities	79.5	82.3
Total liabilities [Member]
Level 3 Assets and Liabilities by Pricing Source:
Total liabilities Internal	79.5	82.3
Total liabilities External
Total liabilities	79.5	82.3
U.S. government and agency [Member]
Level 3 Assets and Liabilities by Pricing Source:
Total assets Internal	25.9	31.7
Total assets External
Fair Value, Total assets, Balance	25.9	31.7	24.0
State and political subdivision [Member]
Level 3 Assets and Liabilities by Pricing Source:
Total assets Internal	58.0	10.4
Total assets External	58.1	38.3
Fair Value, Total assets, Balance	58.0	48.7	13.3
Foreign government [Member]
Level 3 Assets and Liabilities by Pricing Source:
Total assets Internal
Total assets External	8.0	3.1
Fair Value, Total assets, Balance	8.0	3.1	1.4
Corporate [Member]
Level 3 Assets and Liabilities by Pricing Source:
Total assets Internal	649.6	411.6
Total assets External	151.9	102.2
Fair Value, Total assets, Balance	649.6	513.8	337.3
Commercial mortgage-backed (CMBS) [Member]
Level 3 Assets and Liabilities by Pricing Source:
Total assets External	19.3	35.4
Fair Value, Total assets, Balance	19.3	35.4	13.4
Residential mortgage-backed (RMBS) [Member]
Level 3 Assets and Liabilities by Pricing Source:
Total assets External	222.6	268.0
Fair Value, Total assets, Balance	222.6	268.0	216.1
CDO/CLO [Member]
Level 3 Assets and Liabilities by Pricing Source:
Total assets Internal	3.4	61.0
Total assets External	53.4	1.5
Fair Value, Total assets, Balance	3.4	62.5	59.6
Other asset-backed [Member]
Level 3 Assets and Liabilities by Pricing Source:
Total assets Internal	2.8	3.6
Total assets External	77.3	77.6
Fair Value, Total assets, Balance	2.8	81.2	57.7
Inter-company reinsurance derivative asset
Level 3 Assets and Liabilities by Pricing Source:
Total assets Internal		3.5
Total assets External
Fair Value, Total assets, Balance		3.5	(5.5)
Fair value option investments [Member]
Level 3 Assets and Liabilities by Pricing Source:
Total assets Internal	0.8	10.6
Total assets External	21.7	16.9
Fair Value, Total assets, Balance	22.7	27.5	13.8
Total assets [Member]
Level 3 Assets and Liabilities by Pricing Source:
Total assets Internal	740.5	532.4
Total assets External	612.5	543.0
Fair Value, Total assets, Balance	$ 1,353.0	$ 1,075.4	$ 731.1